Protecting ferrous base metal articles against corrosion by treatment with niobium oxide

ABSTRACT

CORROSION OF IRON, STEEL AND OTHER FERROUS BASE METAL ARTICLES IS REDUCED BY A PROCESS IN WHICH SUCH ARTICLES ARE TREATED WITH NIOBIUM OXIDE ADMIXED WITH A WATER MEDIUM HAVING A BASIS PH.

United States Patent 3,809,572 PROTECTING FERROUS BASE METAL ARTICLES AGAINST CORROSION BY TREATMENT WITH NIOBIUM OXIDE Jerome J. Kanter, 12300 Hobart Ave., Palos Park, Ill. 60464 No Drawing. Continuation-impart of abandoned application Ser. No. 882,267, Dec. 4, 1969. This application Feb. 7, 1972, Ser. No. 224,229

Int. Cl. C23f 11/06 U.S. Cl. 117-127 4 Claims ABSTRACT OF THE DISCLOSURE Corrosion of iron, steel and other ferrous base metal articles is ,reduced by a process in which such articles are treated with niobium oxide admixed with a water medium having a basic pH.

This application is a continuation-in-part of application Ser. No. 882,267, filed Dec. 4, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a process for reducing the corrosion of ferrous base metal articles, such as iron, steel, and ferrous base alloys, and more specifically to a process for reducing corrosion of such articles that occurs in the presence of water and/or steam.

The loss of metal due to corrosion has been recognized as an important engineering problem for many years. It is well known that the loss of metal resulting, in common use, from corrosion costs incalculable amounts of time and money.

In addition, corrosion is responsible for many mechanical failures. This has lbeen true particularly of systems subjected to elevated temperatures and pressures.

During recent years, much attention has been directed to the problem of finding corrosion-resistant ferrous base metals for use at elevated temperatures. The stainless steels have been an elfort in this direction. Stainless steels, while they reduce corrosion under ambient conditions are less than completely satisfactory at elevated temperatures and pressures. Moreover, stainless steels are expensive, and the alloying elements needed therefor appear periodically on the scarcity list.

Nevertheless, in the power utility industry, the corrosion problem has become so difficult that resort has been to the expensive stainless steel construction. In spite of this expensive approach, there have been many difficulties and incidents of mechanical failure resulting from stainless steel deterioration at the high temperature service conditions.

In some water coolant systems, for example in connection with air conditioners, various chromate salts are used to reduce corrosion. The water must eventually be discarded, and the cromate salts present a serious Waste disposal problem because of their high solubility. The need for corrosion inhibitors that can readily be separated from water without costly treatments is apparent.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a process for reducing the corrosion of iron, steel, and ferrous base metal articles.

It is another object of the present invention to reduce corrosion of ferrous base metal articles as may occur at 3,809,572 Patented May 7, 1974 temperatures above 212 R, such as are commonly encountered in the boiling water and steam systems used in the power industry.

-It is another object of the invention to reduce corrosion of iron, steel, and ferrous base metal articles that occurs in the presence of water at ambient temperatures.

It is another object of the present invention to provide a water insoluble coating for protecting ferrous base metal articles.

It is still another object of the present invention to provide a method for reducing the formation of iron hydroxide on ferrous base metal articles in the presence of water or steam.

It is still a further object of the present invention to provide a surface barrier for ferrous base metal articles. sufiicient to reduce instances of corrosion.

It is still a further object of the present invention to provide ferrous base metal articles which are suitable for service under corrosion conditions in the presence of moisture, water, and/or steam.

It is still another object of the invention to provide a material that reduces the corrosion of metals, which can be readily separated from water.

It is another object of the invention to provide a corrosion inhibiting additive for water systems that is substantially insoluble in water.

It is a discovery of this invention, corrosion of ferrous base metal articles by moisture, water, and steam is reduced by treating such articles with a niobium oxide admixed with a water medium having an alkaline pH.

Further objects and advantages of the invention will be apparent from an examination of the following description and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT The ferrous base metal articles that are to be protected against corrosion, may be constructed from iron, steel, or ferrous base alloys. The articles may be formed of carbon steels (having a carbon content from about 0.10% by weight to about 0.40% by weight), and particularly the weldable carbon steels, because of their ready availability, comparatively advantageous economic position in the industry, and their otherwise convenient maintainability. The articles may be formed of the stainless steels, such as the 300 series, that are presently being widely adopted for use in the more recent power generators, and the invention is especially applicable to the sensitized parts of stainless steel that are ordinarily the most susceptible to corrosion. The articles may be formed of the cast iron, such as gray iron, malleable iron, or ductile iron, the latter of which is sometimes known as nodular iron.

The ferrous base metal articles most desirably are resistant to corrosion under basic conditions in pH ranges as high as 12, or even as high as pH 14. Other metals having corrosion resistance in that basic pH range may be used in some circumstances, although the ferrous base metals are the most economical for most purposes.

The surface of the ferrous base metal articles to be protected sometimes has an adherent oxide layer, usually of the sub-oxides, such as FeO and Fe O Such oxide films typically occur by mere exposure of the metal article to aid at temperatures below the point at which the oxidation layer matures.

Although formation of the adherent oxide layer is not required in practice of this invention, it has been observed that typically such oxide layers form, if for no other reason, than by the inherent oxidation produced by exposure of ferrous base metals to dry air or water at ambient temperatures. or higher.

It is a discovery of the present invention that ferrous base metal articles are provided with improved resistance to corrosion by treatment with niobium oxide in a water base medium having an alkaline pH. It has been observed that basic pH conditions contribute to the corrosion reducing effect of the niobium oxide in the water medium. The pH of the liquid medium therefore, is desirably maintained in the range of above about 7, and usually in the range from 7.5 to 14. The pH at the upper end of the rangeordinarily does not exceed about 12, and is usually maintained from about 8 to about 12.

The niobium oxide used in the invention is ordinarily the dioxide. Although the mono-oxide of niobium may be employed, it is usually converted to the dioxide under service conditions.

The niobium oxide is insoluble in water and usually is insoluble in the alkaline water medium that is used as a carrier for the corrosion inhibition treatment. The niobium oxide, for this reason, is more conveniently admixed with the water in the form of finely divided particles, such as a powder, which is suspended or dispersed by suitable techniques. It is preferred to have a substantial portion of the niobium oxide of a particle size that will pass a 400 mesh screen, and for some purposes in a particle size classified by levigation and appropriate fluids.

Suitable dispersing agents may be used to keep the niobium oxide particles in suspension in the water medium. Alternatively, it may be desirable to rely upon agitation of the water in the system for dispersion.

The alkaline substances added to the water medium to increase the pH may be suitable well known basic inorganic and organic compounds heretofore used in the power industry to increase the alkalinity of the water. Sodium hydroxide and potassium hydroxide are examples of typical inorganic alkaline materials that may be used to increase the pH of the water medium. Organic compounds, however, are preferred in the power generating plants. Examples of organic alkaline materials used to raise the pH are morpholine and various nitrides.

The amounts of niobium oxide added to the water system to reduce the corrosion may vary. The concentration needed for beneficiation may be as low as 2-10 parts by solids by weight to one billion parts by weight of water. Higher concentrations of the niobium oxide in the alkaline water will ordinarily be used in the practice of the invention, the upper limit being set by the practical limits of suspension in the water medium and/or economic considerations. The lower end of the range will be set by the effective amount needed to produce the desired corrosion inhibition under the conditions of the particular system in question. Ordinarily the niobium oxide will be used in ranges from 0.0001 to 1 part by weight for 100 parts by weight of the water medium.

The corrosion inhibiting method of this invention is readily acceptable for use in various boilers in the utility power industry installations. For example, small amounts of niobium oxide powder may be added to the boiler water, which is normally maintained at an alkaline pH, in order to retard boiler tube fouling. Similarly, the niobium oxide can be added to any process water system in which iron, steel, or ferrous base metal pipe lines are used and corrosion and fouling are serious problems. Water cooling systems, for example in connection with refrigeration units, ordinarily recirculate water that contains chromate salts. It is one of the discoveries of the present invention that the niobium oxide may be substituted for the water-soluble chromate salts in order to reduce corrosion. The niobium oxide, being water insoluble, is advanta- '4 geously separated from the water and recovered so that the water can be discharged to waste or sewage systems without the contaminant. This is a significant advantage in many areas, and provides opportunities for reduction in costs as well as ecological significance.

In many installations, the niobium oxide can be added to the alkaline water medium, and allowed to circulate in the system without further attention. In some large installations, automatic systems may be employed to continuously replenish the niobium oxide in the alkaline water medium.

A practical method is to add an excess of the finely divided niobium oxide to the water medium. A portion of the added niobium oxide is dispersed and/or suspended by suitable techniques, while the other portion can be removed by decanting or filtering, or allowed to remain in the system.

As a general rule, and in the latter example, in particular, as in all good boiler practice, it is desirable tolimit the access of oxygen to the water. Excessive oxygen can cause the formation of adherent black spots of iron. Accordingly, de-oxygenation of the water is usually effected prior to addition of the niobium oxide to the water medium. y

In some systems, it may be desirable to maintain the water medium at minimum temperatures in the range of to 200 F.; during the operation of this process, since upon cooling such liquid mediums typically absorb oxygen and therefore increase the likelihood of corrosion fouling. Alternatively, the system may be not allowed to cool until adequate protective films have formed on the surfaces of the system to guard against oxidation.

It is customary in the nuclear power industry to demineralize the water employed in the generators by ion exchange techniques to concentrations in the range as low as from 2 to 10 parts solids per one billion parts water in addition to deoxygenating the water.

In a nuclear power generator, it may be necessary to add the niobium oxide after the demineralizer in order to allow the niobium oxide to circulate throughout the systembefore returning to the demineralizer.

Other additives may be used in combination with the niobium oxide. The use, for example, of the ammonia, nitrides, or morpholine in conjunction with niobium oxide may be employed to increase the alkalinity of the water. Combinations of various additives may be useful in many instances.

Other techniques may be employed to obtain benefits of the corrosion inhibition of niobium oxide. The niobium oxide in the alkaline water medium can be applied to a ferrous base metal article by brushing, painting or the like to render the article corrosion resistant. The article so treated may be stored or used thereafter in water moisture corrosion conditions.

There are many unusual advantages resulting from the protection of ferrous base metal articles in accordance with the treatment process of this invention. For example, the inclusion of only small quantities of niobium oxide in conventional alkaline boiler water or water cooling systems is sufiicient to reduce the long existing problem of boiler tube fouling.

In addition, the process of this invention lends itself to many applications. The ferrous base metal article may first be fabricated to the desired shape, such as a turbine blade, reaction vessel, or die, and then subjected to the treatment process described hereinabove, in order to prevent corrosion conditions in storage or use. On the other hand, existing ferrous base metal installations can be made corrosion resistant by means of the treatment process by merely adding controlled amounts of niobium oxide to an alkaline water medium, flowing through the installation.

The niobium oxide in an alkaline water medium may be allowed to contact the ferrous base metal for a sufficient time to render the metal resistant to corrosion in the contemplated system. For example, in the boiler of a.

power generating system or the conduits of a water cooling system, the niobium oxide may be allowed to circulate in alkaline water at ambient temperatures and pressures for a time sufiicient to build up a corrosion resistance barrier before the system is subjected to operating conditions.

The following examples serve to illustrate the invention:

EXAMPLE 1 Powdered niobium oxide was added to and admixed with demineralized water. Sufficient sodium hydroxide was added to raise the alkalinity to a pH of about 8.

EXAMPLE 2 Carbon steel plate was introduced into a glass beaker containing the niobium oxide and water admixture prepared as in Example 1. A control was made by introducing a carbon steel plate into a glass beaker containing plain demineralized water. The two beakers were allowed to stand for three days at ambient temperatures, and then compared. The carbon steel plate in the alkaline water containing the niobium oxide showed little evidence of the formation of iron hydroxide. By contrast, the carbon steel plate of the control was covered with iron hydroxide floceulant evidencing corrosion conditions.

EXAMPLE 3 Two glass beakers, one containing the niobium oxide and water admixture with an alkaline pH as prepared in Example 1, and a second glass beaker containing plain demineralized water were heated to boiling to expel all oxygen. The two beakers were then allowed to cool to ambient temperatures and a carbon steel plate introduced into each. The carbon steel plates were observed after one Week. The carbon steel plate contained in the beaker of plain mineral water was pitted and etched. The carbon steel plate in the niobium oxide and water mixture at alkaline pH showed marked corrosion inhibition.

Although the exact nature of the protective mechanism is not fully understood, it is believed that the inclusion of the niobium oxide in a water medium under conditions of controlled alkaline pH, e.g., from 7 to 14, causes a succession of complex oxide compounds to attach to the surface of the ferrous base metal article exposed to the niobium oxide during treatment. A protective, corrosion resistant barrier thereby is formed on the surface of the ferrous base metal article.

At the same time, it is also believed that the niobium oxide when used in the water contained in pipes, boiler tubes or the like, enters into an electrolytic action between the surfaces of the ferrous base metal pipes or tubes and the niobium oxide, which, in turn, reduces the corromen.

In any event, however, the process of this invention is in no way limited to the theory of its operation. Other modes of applying the principles of the invention can be employed.

I claim:

1. In a process for inhibiting the corrosion of a ferrous base metal articles, and which is adapted to reduce the corrosion of said articles in the presence of water or steam, the steps comprising, treating said articles by contacting with a composition consisting essentially of niobium oxide particles admixed in a water medium, and niobium oxde partcles present insaid composition in amounts and contacting said articles for a time sufiicient to provide said articles with improved resistance to corrosion, said water medium having pH above about 7.

2. The process of claim 1 in which said water medium has a pH in the range to about 14.

3. The process of claim 1 in which said water medium has a pH in the range from about 8 to about 12.

4. The article produced by the process of claim 1.

References Cited UNITED STATES PATENTS 6/1971 Bornstein et a1 117l27 8/1972 Lewis et a1 148-63 US. Cl. X.R. l0614; 1486.14

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,809,572 Dated May 7, 1974 Invent r( Jerome J. Kanter It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Specification Column 2, line 65, "aid should be air In the Claims Column 6, line 21, "oxde" should be oxide Column 6, line 22, "partcles" should be particles Column 6, line 25, after "having" insert a Signed and Scaled this thirtieth Day of September 1975 [SEAL] RUTH- C. MaSON C. MARSHALL DANN Allcsrlng ()jjicer (ummissiom'r nj'Pau'nls and Trademarks FORM PO-IOSO (10-69) USCOMM-DC 60376-P69 I & U.S. GOVERNMENT PRINTING OFFICE: I969 O--366-334,

i FORM PO-IOSO (10-69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3-, 809 572 Dated May 7 19 74 Inventoflg) Jerome J. Kanter It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Specification Column 2, line 65, "aid" should be air In the Claims Column 6, line 21, "oxde" should be oxide Column 6, line 22, "partcles" should be particles Column 6, line 25, after "having" insert a Signed and Sealed this thirtieth Day of September1975 [SEAL] Azresr:

RUTH C. MASON .4 Nesting Officer C. MARSHALL DANN (ummissl'um'r qfPalerlts and Trademarks USCOMM-DC 60376-P59 U.S. GOVERNMENT PRINTING OFFICE: I959 O"-366-334 

